Spirit level manufacturers typically offer different categories of spirit levels including inter alia high end contractor grade, low end DIY grade, and the like. There are no international standards defining the different categories of spirit levels but contractor grade spirit levels typically have thicker profiles with larger external dimensions than their DIY grade counterparts, thereby affording greater strength and accuracy. For example, Kapro Industries Ltd, Kibbutz Kadarim, Israel (www.kapro.com), the Assignee of the present invention manufactures a range of spirit levels including inter alia contractor grade Model No. 981 spirit levels with a 25 mm wide 60 mm high box section profile and DIY grade Model No. 781 Light spirit levels with a 20 mm wide 50 mm high box section profile. The former has an about 30% thicker profile than the latter.
Craftsmen typically possess only a single spirit level selected to be suitable for most of their needs but occasionally require a spirit level with a longer working length than the nominal working length of the spirit level in their possession. U.S. Pat. No. 3,648,378 to Thingstad et al., U.S. Pat. No. 4,130,943 to Talbot, U.S. Pat. No. 4,894,925 to Langmaid, U.S. Pat. No. 4,928,395 to Good, U.S. Pat. No. 5,249,365 to Santiago, U.S. Pat. No. 5,433,011 to Scarborough et al, U.S. Pat. No. 5,442,864 to Erman, U.S. Pat. No. 5,617,641 to Aarhus, U.S. Pat. No. 6,041,510 to Huff, U.S. Pat. No. 6,279,240 to Bonaventura, Jr., DE 41 43 268 to Schmidt, and U.S. Patent Application Publication No. US2002/20116833 to Hollenbeck illustrate and describe extension means for selectively extending nominal working lengths of spirit levels.